On e.g. line interface boards provided with line interface circuits and code converters, the signal input terminals of the line interface circuits as well as the signal output terminals of the code converters were earlier related to ground which enabled a direct connection between the signal input terminal of a line interface circuit and the signal output terminal of a code converter. However, modern code converters have a reference voltage for their signal output terminal somewhere between ground and +5 V, most often around 2.25 V, while the signal input terminal of the line interface circuits lies at ground potential. If a line interface circuit whose signal input terminal lies at ground potential, is interconnected with a code converter whose signal output terminal lies at 2.25 V, an offset current is obtained in the line current outputted by the line interface circuit. Upon a polarity reversal of the line current, this offset current will maintain its polarity. Thus, different line currents are obtained for normal and reversed polarity, respectively, of the line current. Also, the offset current can cause other problems.
One way solving this problem is to interconnect a coupling capacitor between the signal input terminal of the line interface circuit and the signal output terminal of the code converter. However, this coupling capacitor has to be large in order for the system not to become unstable.